1. Field of the Invention
The present invention generally relates to a mobile communication system. More particularly, the present invention relates to a method and apparatus for transmitting and receiving control information to randomize inter-cell interference caused by UpLink (UL) transmission in a future-generation multi-cell mobile communication system.
2. Description of the Related Art
In the field of mobile communication technologies, recent study in the area of Orthogonal Frequency Division Multiple Access (OFDMA) or Single Carrier-Frequency Division Multiple Access (SC-FDMA) reveals very promising for high-speed transmission on radio channels. The asynchronous cellular mobile communication standardization organization, 3rd Generation Partnership Project (3GPP) is working on a future-generation mobile communication system, Long Term Evolution (LTE) in relation to the multiple access scheme.
The LTE system uses a different Transport Format (TP) for UpLink control information depending on data transmission or non-data transmission. When data and control information are transmitted simultaneously on the UL, they are multiplexed by Time Division Multiplexing (TDM). If only control information is transmitted, a particular frequency band is allocated for the control information.
FIG. 1 illustrates a transmission mechanism when only control information is transmitted on the UL in a conventional LTE system. The horizontal axis represents time and the vertical axis represents frequency. One subframe 102 is defined in time and a Transmission (TX) bandwidth 120 is defined in frequency.
Referring to FIG. 1, a basic UL time transmission unit, subframe 102 is 1 ms long and includes two slots 104 and 106 each 0.5 ms long. Each slot 104 or 106 is comprised of a plurality of Long Blocks (LBs) 108 (or long SC-FDMA symbols) and Short Blocks (SBs) 110 (or short SC-FDMA symbols). In the illustrated case of FIG. 1, a slot is configured so as to have six LBs 108 and two SBs 110.
A minimum frequency transmission unit is a frequency tone of an LB and a basic resource allocation unit is a Resource Unit (RU). RUs 112 and 114 each have a plurality of frequency tones, for example, 12 frequency tones form one RU. Frequency diversity can also be achieved by forming an RU with scattered frequency tones, instead of successive frequency tones.
Since LBs 108 and SBs 110 have the same sampling rate, SBs 110 have a frequency tone size twice larger than that of LBs 108. The number of frequency tones allocated to one RU in SBs 110 is half that of frequency tones allocated to one RU in LBs 108. In the illustrated case of FIG. 1, LBs 108 carry control information, while SBs 108 carry a pilot signal (or a Reference Signal (RS)). The pilot signal is a predetermined sequence by which a receiver performs channel estimation for coherent demodulation.
If only control information is transmitted on the UL, it is transmitted in a predetermined frequency band in the LTE system. In FIG. 1, the frequency band is at least one of RUs 112 and 114 at either side of TX bandwidth 120.
In general, the frequency band carrying control information is defined in units of RUs. When a plurality of RUs is required, successive RUs are used to satisfy a single carrier property. Frequency hopping can occur on a slot basis when frequency diversity for one subframe is increased.
In FIG. 1, first control information (Control #1) is transmitted in RU 112 in a first slot 104 and in RU 114 in a second slot 106 by frequency hopping. Meanwhile, second control information (Control #2) is transmitted in RU 114 in first slot 104 and in RU 112 in second slot 106 by frequency hopping.
The control information is, for example, feedback information indicating successful or failed reception of DownLink (DL) data, ACKnowledgment/Nagative ACKnowledgment (ACK/NACK) that is generally 1 bit. It is repeated in a plurality of LBs in order to increase reception performance and expand cell coverage. When 1-bit control information is transmitted from different users, Code Division Multiplexing (CDM) can be considered for multiplexing the 1-bit control information. CDM is characterized by robustness against interference, compared to Frequency Division Multiplexing (FDM).
A Zadoff-Chu (ZC) sequence is discussed as a code sequence for CDM-multiplexing of control information. Due to its constant envelop in time and frequency, the ZC sequence offers good Peak-to-Average Power Ratio (PAPR) characteristics and excellent channel estimation performance in frequency. PAPR is the most significant consideration for UL transmission. A higher PAPR leads to a smaller cell coverage, thereby increasing a signal power requirement for a User Equipment (UE). Therefore, efforts should be expanded toward PAPR reduction in UL transmission, first of all.
A ZC sequence with good PAPR characteristics has a circular auto-correlation value of 0 with respect to a non-zero shift. Equation (1) below describes the ZC sequence mathematically.
                                          g            p                    ⁡                      (            n            )                          =                  {                                                                                          ⅇ                                                                  -                        j                                            ⁢                                                                        2                          ⁢                          π                                                N                                            ⁢                                                                        pn                          2                                                2                                                                              ,                                      when                    ⁢                                                                                  ⁢                    N                    ⁢                                                                                  ⁢                    is                    ⁢                                                                                  ⁢                    even                                                                                                                                            ⅇ                                                                  -                        j                                            ⁢                                                                        2                          ⁢                          π                                                N                                            ⁢                                                                        pn                          ⁡                                                      (                                                          n                              +                              1                                                        )                                                                          2                                                                              ,                                      when                    ⁢                                                                                  ⁢                    N                    ⁢                                                                                  ⁢                    is                    ⁢                                                                                  ⁢                    odd                                                                                                          (        1        )            where N is the length of the ZC sequence, p is the index of the ZC sequence, and n denotes the index of a sample of the ZC sequence (n=0, . . . , N−1). Because of the condition that p and N should be relatively prime numbers, the number of sequence indexes p varies with the sequence length N. For N=6, p=1.5. Hence, two ZC sequences are generated. If N is a prime number, N−1 sequences are generated.
Two ZC sequences with different p values generated by equation (1) have a complex cross-correlation, of which the absolute value is 1/√{square root over (N)} and the phase varies with p.
How control information from a user is distinguished from control information from other users by a ZC sequence will be described, by way of example.
Within the same cell, 1-bit control information from different UEs is identified by time-domain cyclic shift values of a ZC sequence. The cyclic shift values are UE-specific to satisfy the condition that they are larger than the maximum transmission delay of a radio transmission path, thus ensuring mutual orthogonality among the UEs. Therefore, the number of UEs that can be accommodated for multiple accesses depends on the length of the ZC sequence and the cyclic shift values. For example, if the ZC sequence is 12 samples long and a minimum cyclic shift value ensuring orthogonality between ZC sequences is 2 samples, multiple accesses is available to six UEs (=12/2).
FIG. 2 illustrates a transmission mechanism in which control information from UEs is CDM-multiplexed.
Referring to FIG. 2, first and second UEs 204 and 206 (UE #1 and UE #2) use the same ZC sequence, ZC #1 in LBs in a cell 202 (Cell A) and cyclically shift ZC #1 by 0 208 and Δ 210 respectively, for user identification. In the illustrated case of FIG. 2, to expand cell coverage, UE #1 and UE #2 each generate a control channel signal by repeating the modulation symbol of intended 1-bit UL control information six times, i.e. in six LBs and multiplying the repeated symbols by the cyclically shifted ZC sequence, ZC #1 in each LB. These control channel signals are kept orthogonal without interference between UE #1 and UE #2 in view of the circular auto-correlation property of the ZC sequence. Δ 210 is set to be larger than the maximum transmission delay of the radio transmission path. Two SBs in each slot carry pilots for coherent demodulation of the control information. For illustrative purposes, only one slot of the control information is shown in FIG. 2.
In a cell 220 (Cell B), third and fourth UEs 222 and 224 (UE #3 and UE #4) use the same ZC sequence, ZC #2 in the LBs and cyclically shift ZC #2 by 0 226 and Δ 228 respectively, for user identification. In view of the circular auto-correlation property of the ZC sequence, control channel signals from UE #3 and UE #4 are kept orthogonal without interference between them.
This control information transmission scheme, however, causes interference between different cells as control channel signals from UEs in the cells use different ZC sequences. In FIG. 2, UE #1 and UE #2 of Cell A use different ZC sequences from those of UE #3 and UE #4 of Cell B. The cross-correlation property of the ZC sequences cause interference among the UEs in proportion to the cross-correlation between the ZC sequences. Accordingly, there exists a need for a method for reducing inter-cell interference caused by control information transmission as described above.